This invention relates to a method of manufacturing an electric resistor suitable for use as a heating element.
Although various types of electric resistors have been proposed some of them are advantageous for certain application but not suitable for other applications and the types of the resistors are determined in accordance with their applications. For example, electric resistors used as heating elements in electric blankets are desired to have different lengths and resistance values dependent upon size, that is the length and width of the blanket. Further, as the blanket is frequently folded or bent, it is desirable that the resistor used therein should be flexible and thin. Such resistors should also be reliable, heat resistant, and able to be manufactured readily at low cost.
Ffor this reason, certain heating elements are made of metal ribbons. However, in order to obtain a heating element of a predetermined dimension it is necessary to use a metal ribbon of a considerable length thereby complicating the manufacturing steps. Further, in order to prepare products of different configuration and electrical capacity it is necessary to prepare metal wires or riibbons of different diameter and length.
A heating element in which fine metal wires are arranged in a mesh and embedded in a plastic sheet has also been developed. In such construction weft or wrap metal wires are used for heating and since the operating voltage is applied across parallely disposed wires there is a limit for voltage control. Moreover, when the spacing between adjacent metal wires is decreased for the purpose of making uniform the temperature distribution, the number of the metal wires is increased. Then, to assure the same power consumption or the rated power capacity it is necessary to reduce the diameter of the metal wire. Accordingly, it is necessary to prepare metal wires of different diameter for the purpose of manufacturing heating elements of different ratings.
Even when a number of wires of different diameters are prepared it is difficult to manufacture products of the desired dimension, size, thickness, configuration and electric capacity. In addition, it is difficult to obtain desired resistance values which vary over a wide range depending upon the application of the product. In other words, with the conventional resistance elements it is difficult to obtain products of any desired configuration and power rating unless an elaborate manufacturing facility is installed. The same problem exists in resistors utilized for other purpose than heating.
Terminals for connecting the resistors to other circuit components have been secured to the opposite ends of the resistors by mechanical means or welding. However, such methods are not only troublesome but also cannot eliminate the contact resistance between the resistors and the terminal fittings.